Electrodeposition of zinc on magnesium and its alloys



1957 H. K. DE LONG ELECTRODEPOSITION OF ZINC ON 'MAGNESIUM AND ITS ALLOYS Filed June 20, 1956 "I ll I H INVENTOR. Herber/ K. Dela/7y I flTTORNEYS nited States Patent ELECTRODEPOSITION OF ZINC N MAGNESIUM AND ITS ALLOYS Herbert K. De Long, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application June 20, 1956, Serial No. 592,554

6 Claims. (Cl. 204-55) The invention relates to a method of providing zinc plating on articles of magnesium and magnesium-base alloys containing upwards of 85 percent magnesium. It more particularly relates to a method of providing zinc plating on articles of the aforementioned metals by the direct electrodeposition of zinc in a single step.

Magnesium and its alloys are difficult to provide with a metallic coating. Although a number of methods of coating magnesium and its alloys have been proposed, coatings produced by such methods have not been wholly satisfactory. Particularly unsatisfactory have been proposed methods of zinc-coating such metals by electrodeposition.

For many uses of magnesium articles it is desirable that they be provided with a zinc basis coating and one or more additional electrodeposited coatings, e. g., platings of nickel, chromium, and/or copper. Heretofore, the zinc electroplatings on magnesium articles have not been sufficiently thick and adherent to serve as a basis coating for subsequent fully satisfactory electrodepositions of such additional coatings.

It has been proposed to electrodeposit zinc on articles of magnesium and its alloys either by chemically depositing a first layer of zinc and electrodepositing a second and discontinuous layer of zinc thereon, or by electrodepositing Zinc followed by a subsequent step of subjecting the zinc-coated article to sintering or heat treatment from about 600 to about 800 F. for upward to an hour. Among the objections to such methods are the inconven ience and cost of the additional steps. Furthermore, it has heretofore been necessary to sand blast magnesium pieces or articles prior to zinc coating. The purpose of the sand blasting is to provide for a mechanical interlocking or interdigitation of the zinc with the magnesium basis metal. A sand blasted surface usually requires a heavier deposit of the coating metal to completely fill the microscopic indentations and the larger irregularities resulting from the preferential attack of the sand on the surface to attain a smooth coating. Sand blasting wastes magnesium and an unnecessarily heavy coating wastes zinc. The method of plating magnesium with zinc according to the invention renders sand blasting unnecessary because a sufficiently strong chemical bond is formed between a smooth unblasted magnesium surface and the deposited zinc.

It is a desideratum in the art to provide a method by which the magnesium or magnesium-base alloy articles may be given an improved protective coating of zinc by direct electrodeposition unaccompanied either by prior zinc deposition by chemical means or by a subsequent sintering treatment.

Accordingly, it is a principal object of the invention to provide a method of fulfilling this need. A particular object is to provide a method of plating magnesium and magnesium-base alloy articles with zinc which is sufficiently adherent, thick, and impervious to provide an adequate basis coating for further metal coatings deposited thereon.

2,811,484 Patented Oct. 29, 1957 To the accomplishment of the foregoing and related objects, said invention then comprises the steps herein fully described, and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be applied.

Broadly stated the method contemplated by the invention comprises zinc plating a cleaned article of magnesium or magnesium base alloy in an electroplating solution comprising an aqueous solution of a zinc salt, a pyrophosphate, a sulfate, and a fluoride, either with or without a carbonate or a bicarbonate, whereby a zinc coating is electrodeposited on the magnesium article which is adherent and substantially impervious.

The zinc plating thus obtained according to the invention may be used as a single protective coating, or as a basis coating for other metallic coatings, particularly those deposited thereon by electrodeposition as for example from an alkaline cyanide'aqueous electroplating bath. These electroplates maybe used as the basis for ,still other metallic coatings such as those which cannot readily be directly deposited on zinc or magnesium.

Prior to carrying out the method of the invention, the surface of the magnesium article is cleaned in a manner to leave the surface free from contamination and in such condition that the magnesium constitutes a receptive site for the deposition of zinc formedby the reaction of the ingredients contained in the electroplating solution. The surface of the article may be prepared for zinc plating by cleaning, such as machining, wire brushing, bufiing, and the like, or by a chemical treatment with solvents, e. g., an aqueous solution of sodium hydroxide and sodium carbonate and/ or pickling agents, e. g., an aqueous solution of chromic acid and a strong mineral acid which may also contain some hydrofluoric acid. The method used is chosen in accordance withthe condition of the surface to be cleaned and the degree of smoothness desired. Chemical cleaning is usually preferred. However, the surface chemically cleaned usually becomes coated very quickly with an undesirable thin oxidic film and requires a subsequent preparatory treatment.

The oxidic film, if formed, should be removed before proceeding with the method of the invention. A convenient and economical treatment for removing the oxidic film is to immerse the chemically cleaned article in a bath comprising an aqueous solution of phosphoric acid and an ammonium acid fluoride. The aqueous solution is made by dissolving about 200 g. of '85 percent by weight aqueous phosphoric acid and about g. of dry ammonium acid fluoride in enough water to make a liter of solution. I

In carrying out the method of the invention, the aforementioned electroplating solution is prepared -and the magnesium article is made the cathode either by immersion therein or by employment of an electrochemical brush-plating apparatus, wherein the brush is made the anode and the magnesium article to be plated is made the cathode; electrolysis is effected by dipping the brush into the solution and applying the brush to the article.

The electroplating solution is prepared by admixing the following active molecular fractions within the range set out in grams per liter of water:

or a bicarbonate fraction (H003) has been found to improve the performance of the bath. The improvement is likely due to the ease with which the pH value of the solution may be controlled by addition of either the carbonate or bicarbonate group and their beneficial effect upon the conductivity. The carbonate group may be present up to 110 g./ liter of solution, but to be noticeably effective ought to be present at least in the amount of 2.5 g./ liter. The bicarbonate may be present up to 30 g./ liter of solution.

The pyrophosphate is preferably added as the alkali salt of which the pyrophosphate fraction (P207) makes up between 110 and 222 g./liter of solution. used herein, is a postive element or radical selected from the class consisting of ammonium and the metals of group I of the periodic table. The zinc is preferably added as the sulfate or pyrophosphate salt of which the zinc fraction (Zn) makes up between 4.3 and 8.6 g./liter of solution. The fluoride is preferably an alkali fluoride of which the fluoride fraction (F) makes up between 3.9 and 4.6 g./liter of solution. The sulfate is preferably an alkali sulfate of which the sulfate fraction (S04) make up 2 to 47 g./liter of solution.

superior adhesion of the zinc plating which is attained when potassium salts are used.

A carbonate or a bicarbonate, preferably as an alkali carbonate or bicarbonate, may be added for the purposes of controlling pH and to improve conductivity. The preferable weight of the carbonate group (CO3) is between 60 and 87 g. and that of the bicarbonate fraction (HCOs) about 20 g. per liter of solution. The presence of the carbonate also appears to inhibit corrosion of the Zinc anode and to keep it bright and free of polarization effects during the plating operation. Amounts of carbonate in excess of the operable limits set out above may be present but such amounts tend to slow the rate of deposition and result in a thinner zinc plating. The presence of the alkali bicarbonate with the carbonate, appears to enhance the adhesion of the zinc coating.

The solution may have a pH between 10.2 and 12.0, but more desirably between 10.2 and 10.8, but a pH value of 10.5 is distinctly preferred. As the pH approaches 12.0, deposition is slower and the quality less desirable. If the pH is permitted to drop to 10.0 or lower, galvanic or immersion deposition may take place which is detrimental to the electrodeposition. It is recommended that the pH be maintained within the preferred range by addition of a carbonate to raise the pH or of a bicarbonate to lower the pH. For example, at 25 C. a 0.1 N solution of sodium carbonate has a pH of 11.6 and a 0.1 N solution of sodium bicarbonate has a pH of 8.4. Pyrophosphoric acid may also be advantageously added in suflicient amount to lower the pH to the desired value.

The solution may be maintained at a temperature between 70 and 190 F. but is usually maintained between 115 and 125 F. A substantially pure zinc piece is made the anode and the magnesium article to be coated is made the cathode of a direct current. A current density on the cathode between 3 and 30 amp/sq. ft. may be employed, but one between and 15 amp/sq. ft. is preferred. The time required for electrodeposition depends on the thickness of the zinc coating desired and upon such operating conditions as current density and upon the composition, temperature, and pH value of the solution. An electroplating period of to minutes is usually employed.

The magnesium article is then removed from the solution and rinsed well with water. A plating of zinc is thus formed which adheres very well, is free of blisters and other blemishes, and affords a high degree of protection for the magnesium. The zinc plating is particularly valuable as a basis coating for additional electrodeposition of other metals, e. g., copper, nickel, or chromium thereon. Such additional electrodepositions may be accompanied by making the zinc-coated magnesium article a cathode in an appropriate electroplating solution.

An alkali, as i The alkali element usually used is potassium because of its greater solubility and the A. Specific examples of zinc-plating magnesium articles according to the invention are as follows:

Example 1 A magnesium alloy die cast panel 2" x 6 x Ms, having the composition: 90.2% Mg, 9.0% Al, 0.2 Mn, and 0.6% Zn, was degreased in an alkaline bath consisting of 3 oz. of sodium carbonate crystals and 2 oz. of sodium hydroxide in a gal. of water at 94 C. for 5 min. The panel was immersed for 1 minute at 28 C. to brighten and smooth the surface and remove gross oxidation product in an aqueous pickle solution consisting by weight of:

28.0% of chromium trioxide 2.5% of 70% aqueous nitric acid 0.8% of 52% aqueous hydrofluoric acid Balance water The panel was rinsed in cool water and then immersed for 2 minutes at 28 C. in an activating bath consisting by weight of 20% of aqueous phosphoric acid and 10% ammonium acid fluoride in water.

The panel was again rinsed in cool Water and then made the cathode in the novel electroplating solution of the invention previously prepared by dissolving the following ingredients in sufficient water to make a liter of solution:

Grams/Liter of Ingredient Aqueous Solution K4P2O7-3H2O a 400. 0 Z112P2O7 a 20. 0 NazSQg- 20. 0 KF 1 l2. 5 NazCOa 80. 5

The pH value of the bath was 10.5 and the temperature 120 F. The current density of the cathode was 10 amp/sq. ft., D. C. The electroplating was continued for 10 minutes after which the panel was removed and rinsed well with cool water. A zinc plating was obtained on the magnesium panel which showed no effects of blistering during the electrolysis. The panel was tested for the excellence of adhesion of the zinc plating by the knife test and burnishing test. The knife test consists essentially of scratching a large cross on the surface with a sharp pointed blade so as to completely penetrate the electrodeposit and thereafter probing the exposed edges of the electrodeposit along the scratch, attempting thereby to lift the electrodeposit from the basis metal. No flaking or peeling was obtained. The burnishing test consists essentially of rubbing the electrodeposit vigorously with a steel rod (pencil size having a pointed end) on a localized area. Poor adhesion causes the electrodeposit to be lifted from the basis metal. The panel was also bent and flexed to ascertain the extent of resistance to cracking and checking of the plating. No cracking or checking was observed.

The zinc-plated panel was then made the cathode of a chrome-plating bath at a temperature of F. consisting of the following aqueous composition:

300 g. CrOa 3 g. 98% H2304 Water to make 1 liter of solution The current density was 1.5 amp/sq. in. for the first 10 seconds and 1 amp/sq. in. for the balance of 4 minutes.

After 0.0002 chromium was formed on the article, it was removed and washed with cool water. The chrome coating was subjected to the same tests as the basis zinc plating described herein above. It was found to be highly adherent. A 0.0002 thick coating of chrome is an especially thick coating and shows a tendency to peel before a thinner coating under comparable conditions shows such tendency.

Example 2 Example 1 was repeated except the copper electroplating bath below was used instead of the chrome bath of Example 1 following the zinc electrodeposition according to the invention. The copper electroplating bath was made by forming a solution of the following ingredients:

Ingredients Grams/Liter of Solution OuCN 41. 25 NaCN 51. 0 N 82C 0': 30. 0 KNaO4HrOa-4HrO (Rochelle Salts) 46. 0 Water to make 1 liter of solution.

Example 3 The procedure followed in this example was that of Example 2 except that two further successive electrodepositions of metals were made on the copper as follows:

The copper-plated, zinc-plated magnesium panel pro duced as in Example 2 was made the cathode in a standard Watts type nickel bath and electroplated with nickel to 0.001 in thickness.

The nickel-plating Watts type bath was made up by forming a solution of the following ingredients:

Ingredient Grams/Liter Solution NiSOHiHzO 240 NiCLz-6H2O 45 H313 O 1 30 Water to make a liter of solution.

The pH was held at about 3.5 and the temperature was held at about 132 F. A smooth adherent nickel plating was obtained on the copper plate as determined by the tests applied to the zinc plating described in Example 1 above.

The nickel-plated panel was then removed, washed well with cool water, and made the cathode of a chromium electroplating bath for 4 minutes at a temperature of 100 F. The current density was 1.5 amp/sq. in. the first seconds and 1 amp/sq. in. for the remainder of the plating period. The bath had the same composition as the chromium bath of Example 1, viz., 300 g. of CIO3 and 3 g. of H280; dissolved in sufficient water to make a liter of the electroplating solution. The plated panel was then removed and washed with cool water.

The magnesium base alloy panel now had four successive electrodepositions on its surface, viz., zinc, copper, nickel, and chromium. The resulting plate was hard, bright, and smooth and adhered extremely well as shown by the tests set out in Example 1 above.

Example 4 In this example, zinc plating according to the invention was carried out by employing the brush apparatus shown in the drawing. Figure 1 shows a 0.04" thick magnesium base panel 10, having the composition: Mg 96%, Al 3%, and Zn 1%, upon which area 1 is shown cleaned preparatory to being plated. Figures 2 and 3 Show the principal parts of the apparatus before assembly. Figure 4 shows the brush assembled and in contact with the magnesium base panel being plated. Brush 4 is provided with bristles 5, and insulating handle 6 which has an elongated passageway therethrough for insertion therein of electrical conductor 7. Zinc piece 8 with threaded means for engagement with conductor 7 after its insertion in handle 6 is the anode of the brush when in operation. Conductor 7 is shown attached by threaded engagement to a D. C. positive lead-in line 9. Negative lead-in line 2 is shown attached to panel 10 by means of spring-loaded clamp 3. The brush apparatus shown in thedrawing is conventional and forms no part of this invention.

The zinc plating solution of the invention for brushelectroplating was prepared by admixing the following ingredients:

KF Ammonium alginate Water to make 1 liter of solution.

In carrying out brush-electroplating, the solution was maintained at 28 C. and the pH maintained at 10.5 by occasional additions of NaOH as required. The solutions was kept in agitation by a variable speed electric stirrer. The bristles 5, having positioned therein the piece of substantially pure zinc 8, were dipped into the electroplating solution and applied as shown in Figure 4 to the cleaned area of the magnesium base panel being plated. The brush was moved back and forth while in contact with the panel so as to spread evenly the zinc which was deposited continuously from the electroplating solution retained among bristles 5. A current density of 10 amps/sq. ft. at the cathode was produced when the brush containing the electroplating solution in the interspaces among the bristles was applied to the magnesium article. The brush was dipped as often as required to keep the interspaces sufficiently filled with solution for good operation. The operation was continued for 15 minutes.

Thereafter the panel was disconnected and rinsed well in cool water. A medium gray-colored zinc plating was formed on the panel which was adherent and uniform as shown by bending, knife peeling, and burnishing described in Example 1 above.

The ammonium alginate produced a paste-like consistency which permitted easier application of the zincplating solution and also produced a brighter zinc plate.

In brush-electroplating the voltage is usually between 6 and 20 volts and the amperage between 0.3 and 6.0 amperes as measured between the anode and cathode. The temperature may vary between 60 and 72 C. The work area is conveniently at about room temperature.

Example 5 4 x 4" photoengraved magnesium-base printing plate having the composition Mg 96%, Al 3%, and Zn 1% was prepared for photoengraving according to the conventional manner of coating with a photo-sensitive enamel, photo-transferring the desired image onto the plate and subjecting the plate to an etching acid preparation to preferentially etch the image in relief on the plate. Thereafter, the raised areas of the plate or tops which still retained the enamel were rubbed with charcoal to remove the enamel. The plate was then degreased as in Example 1 and thereafter activated with the HaPO4NH4HFz bath set out in Example 1.

The printing plate was then zinc-plated followed by chrome-plating as in Example 1. An excellent plated 7 surface'of chromium was thereby obtained on the zincplated magnesium base printing plate. It was hard and adherent as shown by the tests described in Example 1. Having thus described my invention, what I claim and desire to protect by Letters Patent is:

1. The method of zinc-plating an article composed of at least 85 percent magnesium which comprises making the article to be plated the cathode in an aqueous electroplating solution having a pH between 10.2 and 10.8 at a temperature between 70 and 190 F., said solution containing per liter a pyrophosphate of an alkali of which the pyrophosphate fraction comprises from 40 to 310 grams, a zinc salt of which the zinc fraction comprises from 4.3 to 19.35 grams, a fluoride of an alkali of which the fluoride fraction comprises from 2.5 to 6.5 grams, and a sulfate of an alkali of which the sulfate fraction comprises from 2 to 47 grams.

2. The method of claim 1 wherein the electroplating solution contains a salt selected from the group consisting of carbonates and bicarbonates of an alkali of which the carbonate comprises up to 110 grams and the bicarbonate comprises up to 30 grams per liter of solution.

3. The method of claim 2 wherein the zinc salt is selected from the class consisting of zinc pyrophosphate and zinc sulfate.

4. The method of claim 1 wherein the alkali is potassium.

5. The method of claim 1 wherein the said solution is applied to the cathode with a brush having bristles, the interspaces among said bristles being made to contain said solution.

6. The method of claim 5 wherein said solution contains ammonium alginate in amount up to 1.9 percent by weight of the solution.

References Cited in the file of this patent UNITED STATES PATENTS 1,791,082 Bianco Feb. 3, 1931 2,437,612 Osborg Mar. 9, 1948 2,488,246 Stareck et al. Nov. 15, 1949 2,511,952 Stareck et a1 June 20, 1950 

1. THE METHOD OF ZINC-PLATING AN ARTICLE COMPOSED OF AT LEAST 85 PERCENT MAGNESIUM WHICH COMPRISES MAKING THE ARTICLE TO BE PLATED THE CATHODE IN AN AQUEOUS ELECTROPLATING SOLUTION HAVING A PH BETWEEN 10.2 AND 10.8 AT A TEMPERATURE BETWEEN 70* AND 190*F., SAID SOLUTION CONTAINING PER LITER A PYROPHOSPHATE OF AN ALKALI OF WHICH THE PYROPHOSPHATE FRACTION COMPRISES FROM 40 TO 310 GRAMS, A ZINC SALT OF WHICH THE ZINC FRACTION COMPRISES FROM 4.3 TO 19.35 GRAMS, A FLUORIDE OF AN ALKALI OF WHICH THE FLUORIDE FRACTION COMPRISES FROM 2.5 TO 6.5 GRAMS, AND A SULFATE OF AN ALKALI OF WHICH THE SULFATE FRACTION COMPRISES FROM 2 TO 47 GRAMS. 